Observation of two critical points linked to the high-field phase B in CeCu2Si2

Abstract

We present thermal expansion and magnetostriction measurements on a CeCu2Si2 single crystal of A/S-type up to 17.9 T magnetic field applied along the crystallographic a-direction (∆L a H) and down to 0.015 K temperature. We identify clear thermodynamic anomalies at the superconducting transition Tc and at two second order transitions TA,B into ordered phases A and B. Our measurements establish for the first time the boundary of phase B at high field and low temperature. No evidence for additional high field phases above B is found up to the maximum field. We speculate based on our experimental results that i) phase B is similar to phase A of spin-density wave type and ii) the first order phase transition between A and B is caused by Fermi surface reconstruction. We furthermore identify a new quantum critical point at Hc 17 T, where TB is suppresssed to zero, and a bicritical point at (0.35 K, 7.0 T), where phase lines TA(H) and TB(H) meet.